drm/i915: Polish sprite plane register definitions

Group the sprite plane register definitions such that everything
to do with the same register is in one place.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240516135622.3498-13-ville.syrjala@linux.intel.comReviewed-by: Jani Nikula <jani.nikula@intel.com>